(1) Field of the Invention
The present invention relates to a return device for retracting a movable attachment device that enables an aircraft fitted with the attachment device to carry an external load.
The technical field of the invention is thus the field of systems that enable an external load to be fastened to an aircraft, and in particular to an aircraft having a rotary wing such as a rotorcraft or a helicopter.
(2) Description of Related Art
Conventionally, an aircraft has a structure referred to for convenience as a “carrier structure” for carrying an external load that is independent of the aircraft. The carrier structure may comprise frames secured to the fuselage of the aircraft, a covering, or indeed a transport snub wing.
The aircraft can then be provided with an attachment device connected to the carrier structure for carrying an external load. Generally, such an attachment device is attached under the carrier structure of the aircraft.
By way of example, the attachment device may be provided with hook-type attachment means. A sling is then attached to the hook in order to make it possible to transport external load.
The attachment device may also include a fastener device for suspending the attachment means from the carrier structure. The fastener device may be:
a first fastener device known as a “sling”, which first device is of the universal joint type having two mutually orthogonal pivot axes;
a second fastener device known as a “swing”: the second device comprising a pyramid-shaped frame suspended from the carrier structure, e.g. by suspension cables known as “pendants”; or
a third fastener device making use of a beam to which the hook is attached, the beam generally being fastened to the carrier structure at at least two points.
By way of example, Document U.S. Pat. No. 3,044,818 describes a fastener device having a pyramid-shaped frame. The pyramid-shaped frame may comprise a base having three or four sides, with bars rising from the base towards a top of the frame. The attachment means are situated under an opening provided in a fuselage.
On an aircraft having a lift rotor, the attachment device is advantageously arranged close to the rotor axis of the lift rotor so as to limit the attachment device generating disturbances to the behavior in flight of the aircraft and thus to piloting it. In the same manner, the fuel tanks of an aircraft may also be arranged in the proximity of the rotor axis. Consequently, in flight, an attachment device may be situated under a tank of the aircraft.
Under such circumstances, the aircraft may include a return device tending to move the attachment device away from the tanks in the absence of an external load, or away from any other member that needs to be protected. Thus, the return device tends to avoid any risk of a sensitive member of the aircraft being punctured by the attachment device in the event of a crash.
Such a return device may comprise at least one resilient return means exerting a return force in order to move the attachment device away from a sensitive zone in the absence of an external load. The resilient return means may be of the bungee cord type, and may for example be connected to the fastener device carrying the attachment means.
When an operator suspends an external load from the attachment device, the resilient return means then tend to stretch. The external load together with the attachment device thus tend to become vertically aligned under the effect of gravity, being positioned in a position that is referred to for convenience as the “working” position.
In contrast, when the external load is removed, the resilient return means can retract so as to move the attachment device away from the working position so as to position the attachment device in a position that is referred to for convenience as the “storage” position. By way of example, the storage position is selected so as to limit any risk of a fuel tank being punctured by the attachment device in the event of the aircraft crashing.
The resilient return means exert a large return force on the fastener device in order to hold it in the storage position in spite of the accelerations to which the aircraft can be subjected during a mission. The return force also increases as a function of the amount of stretching of the resilient return means, and thus of the external load being carried and also of the forward speed and/or the acceleration of the aircraft.
Under such circumstances, the return force is large in order to hold the attachment device in the storage position. This return force is even greater when a load is present, since the resilient return means are then in a stretched position. Under such circumstances, the resilient return means can disturb the balance of the external load carried in the working position by generating a return force that is not negligible.
Furthermore, when the external load is released, the return force leads to the attachment device returning suddenly and quickly to the storage position. Abutments may be installed to avoid an impact against the carrier structure. Nevertheless, repeated sudden returns of the fastener device can end up weakening such abutments. Furthermore, since the return force is large, the attachment device is subjected to acceleration that is likewise large and is returned so as to reach its storage position at high speed. This process requires the abutment system to be capable of absorbing the energy that corresponds to this high impact speed.
In addition, in order to fasten an external load, an operator needs to exert a considerable traction force on the attachment device so as to combat the return force exerted by the resilient return means. The further the operator moves the attachment device away from its storage position, the greater the force that the operator needs to deliver.
To remedy those drawbacks, it is possible to use return means that extend over a long length so as to maximize the amount of stretching they can accept in the elastic range.
Attachment devices may include dampers, blocking means, or indeed devices that are activated as a function of the position of the fastener device.
Document U.S. Pat. No. 3,044,818 describes an attachment device having a fastener device with a pyramid-shaped frame. The pyramid-shaped frame may have a base with three or four sides, together with bars rising from the base to a top of the frame. The attachment means are situated under an opening arranged in a fuselage.
Document U.S. Pat. No. 3,265,336 describes an attachment device forming a flexible pyramid-shaped structure having a cable and a plurality of pulleys. Such an attachment device is nevertheless constantly arranged under the fuselage of an aircraft and can not be retracted.
Document FR 3 008 385 describes an aircraft having an attachment device and a return device for retracting the attachment device. That attachment device includes an attachment member suitable for carrying a load. The return device includes resilient return means and a force-reduction member provided with a plurality of bars hinged together in pairs to form lever arms and to apply a non-linear force to the attachment device. Furthermore, the return device includes a tie connecting the force-reduction member to the attachment device, and the resilient return means are formed by a spring that is fastened at one of its ends to a bar of the force-reduction member and that is connected to an aircraft at another end.
In the storage position, the force-reduction member is unfolded so as to form a maximum angle (200) between the various lever arms. The force-reduction member thus enables a large force to be exerted on the tie and thus on the attachment device.
In contrast, in the working position, the force-reduction member is folded up so as to form a minimum angle (200) between the various lever arms. The force-reduction member thus enables a smaller force to be exerted on the tie and thus on the attachment device.
Consequently, the return device exerts a force on the attachment device that is at a maximum when the force-reduction member is unfolded. That force enables the attachment device to be held in its retracted storage position in spite of the high accelerations that can be generated during flight of the aircraft.
Conversely, the return device exerts a force on the attachment device that is reduced when the force-reduction member is folded up. Under such circumstances, the return device has a limited effect on the attachment device in the deployed working position.
Furthermore, the variation in the return force on going from a retracted storage position of the attachment device to a working position does not vary linearly. On the contrary, the return force decreases very quickly from the retracted storage position going towards a minimum value. Under such circumstances, the force that an operator needs to deliver in order to oppose the return force when manipulating the attachment device is significant above all when the attachment device is in its retracted storage position.
Finally, the force-reduction member leads to a reduced retraction speed for the movement of the attachment device from the deployed working position to the retracted storage position.